proper "404" screen on details page

[danoswaltCL]

Currently if you paste in a url do a /detail page with a bad id, the page will show a brief error toast, and the spinner just spins. It would be better to actually show something, i coded up something that looks like this, for feature flag, experiment, or segments pages. If this design is okay and we agree this would be a better way to handle it, i can push this up as a PR: